(1) Field of the Invention
The present invention relates to an improvement in the package for housing semiconductor elements of a high-density large-size semiconductor integrated circuit such as LSI. More particularly, the present invention relates to a package for housing semiconductor elements, which is constructed by using a specific mullite sintered body as the insulating substrate.
(2) Description of the Prior Art
A package for housing elements of a semiconductor integrated circuit such as LSI or a multi-layer circuit board having elements of a semiconductor integrated circuit mounted thereon has heretofore been fabricated by using alumina (Al.sub.2 O.sub.3) having an excellent electric insulating property as the substrate and forming an electric wiring of a metal such as tungsten (W), molybdenum (Mo) or manganese (Mn) on the surface of the substrate.
However, in a package or multi-layer circuit board fabricated by using alumina as the substrate, the propagation speed of a signal transmitted through the electric wiring is low because the dielectric constant of alumina constituting the substrate is as high as 9 to 10 (at room temperature and 1 MHz), and therefore, it is impossible to house or mount elements of a semiconductor integrated circuit, in which the signal propagation speed is recently increased, in or on a package or multi-layer circuit board comprising alumina as the substrate.
Furthermore, with recent increase of the density or integration degree in semiconductor integrated circuit elements, the sizes of the elements per se are increased, and in the case where such semiconductor integrated circuit elements are practically arranged in the conventional package or multi-layer circuit board, since the thermal expansion coefficient of the elements is greatly different from that of the package or circuit board, on application of heat for the practical arrangement, the package or circuit board expands more greatly than the semiconductor integrated circuit elements, with the result that such troubles as breaking of the semiconductor integrated circuit elements and peeling of the elements from the package or multi-layer circuit board take place.
Accordingly, in order to overcome these defects of the conventional package or multi-layer circuit board comprising alumina as the substrate, we tried to use, as the substrate of the package or multi-layer circuit board, mullite having a thermal expansion coefficient of 4.0 to 4.5.times.10.sup.-6 /.degree.C. (from room temperature to 400.degree. C.), which is substantially equal to the thermal expansion coefficient of silicon constituting semiconductor integrated circuit elements, that is, 3.0 to 3.5.times.10.sup.-6 /.degree.C. (from room temperature, to 400.degree. C.), and having a dielectric constant as low as 6.5 to 7.0 (at room temperature and 1 MHz).
However, since the sintering property of mullite is lower than that of alumina, the relative density of the obtained sintered body is low and about 85% of the theoretical density and the sintered body has many pores. Accordingly, in the case where the sintered body of mullite is used as the substrate of a package or multi-layer circuit board and an electric wiring is formed on the surface, a problem of breaking of the wiring because of the presence of the pores is found to rise. Moreover, since the sintered body of mullite has many pores, the heat-dissipating property is poor and therefore, it is impossible to dissipate heat generated by the mounted semiconductor integrated circuit elements effectively into the open air, with the result that the semiconductor integrated circuit elements are caused to peel off from the package or multi-layer circuit board by the heat history owing to heat generated by the elements per se.
In the case where semiconductor integrated circuit elements are connected to external lead terminals by an automatic wire bonder, registering target marks on the package or multi-layer circuit board are detected by a sensor of the automatic wire bonder to effect registration. However, since the color contrast between the golden target mark and the white mullite substrate is low, there arises a problem of occurrence of erroneous registration.
Accordingly, in order to overcome the foregoing defects, we tried to obtain a black sintered body of mullite by adding an additive such as iron oxide (Fe.sub.2 O.sub.3), manganese oxide (MnO.sub.2) or titanium oxide (TiO.sub.2) to mullite and sintering the mixture in the open air.
However, it was found that a black mullite sintered body obtained by adding an additive as mentioned above and carrying out sintering in the open air is defective in that a metallized metal layer composed of a high-melting-point metal such as tungsten, molybdenum or manganese cannot be formed on the surface simultaneously with sintering.
In the case where a metallized metal layer as mentioned above is formed simultaneously with sintering in a reducing atmosphere, titanium oxide TiO.sub.2 in the additive is reduced to a lower-order oxide TiO.sub.2-x and the insulating property of the mullite sintered body is degraded by the formed oxygen lattice defects. Moreover, iron oxide (Fe.sub.2 O.sub.3) and manganese oxide (MnO.sub.2) are reduced to metallic iron (Fe) and metallic manganese (Mn), respectively, and these metals form a solid solution with the tungsten metallized metal layer to lower the melting point. Accordingly, the fusion state is brought about at a temperature lower than the sintering temperature and it is impossible to form the metallized metal layer on the sintered body with a high adhesion strength.